> Getting a 16 bit converter that fast, approaches the limit of available > technology.
Right. At least at reasonable cost. > These converters are pipeline architecture and have better than 90 db dynamic > range. Theoretically the dynamic range (ADC overload to noise floor) of an ADC is a little better than 6 dB * number of bits, which would be over 96 dB for a 16-bit ADC. But they are never that good in practice. I believe 90 dB is way optimistic, especially when you take into account the analog circuitry that is needed in front of the ADC. For example, the 16-bit LTC2107 from Analog Devices has 80 dBFS signal-to-noise ratio. Note that is a theoretical figure in that it assumes no degradation at all by external circuitry. This state-of-the-art device draws 1.28W of power and costs $99 in 1000-piece quantity. > Looking at S units, there is 88 db from S1 to 40 over S9 before any > attenuation or > AGC is applied prior to the ADC. >> K3S K4HD Feature >> 24 16 ADC width As others have noted, that's an "apples and oranges" comparison because the 16-bit direct-sampling ADC is way oversampled compared to the 24-bit ADC at baseband or low-frequency IF. The S/N ratio of an ADC is improved by 10 * log(sample_rate/receive_bandwidth) dB For example, if the sample rate is 130 MHz and the receive bandwidth is 500 Hz, that's about a 54 dB improvement. If the ADC dyanamic range, including all external circuitry, is (let's say) 75 dB then the 500 Hz dynamic range is 129 dB. I think with current technology at reasonable cost, the high 120's dB is about the best you can do with a direct-sampling receiver and many don't do that well. By constrast, I believe the blocking dynamic range of a K3S is about 150 dB in a 500 Hz bandiwdth. I assume the K4 high-dynamic-range option will be similar. Whether you need the extra 20-30 dB of blocking dynamic range depends on your situation and what you plan to use the radio for. Alan N1AL On 2019-05-29 09:37, W2xj wrote: > The K4 is a direct sampling radio. It has no IF. It samples directly to IQ > baseband and performs all filtering and processing in the digital domain. > That requires a very high speed ADC. In this case it is just above the > Nyquist limit for 6 meters. Getting a 16 bit converter that fast approaches > the limit of available technology. These converters are pipeline architecture > and have better than 90 db dynamic range. Looking at S units, there is 88 db > from S1 to 40 over S9 before any attenuation or AGC is applied prior to the > ADC. The problem comes when there are signals exceeding that dynamic range > like working someone who might be S1 or S2 while someone else is transmitting > nearby as a direct conversion radio has no front end selectivity. As Eric > already explained, this is the reason for the K4HD option. This appears to > essentially be a K3S superhet single conversion front end that feeds an > approximate 8 MHz IF into the ADC. It would then provide a very narrow band > receptio! n o > ption for the K4. > > Looking at K3s architecture, the ADC is a PCM1804 which is a delta sigma > converter which is basically a one bit ADC clocked at 768 times the sample > rate or somewhere between approximately 24 and 36 MHz depending on what > sample rate Elecraft selected. In the mode used in the K3S it can sample up > to 48 KHz although there are other modes permitting up to 192 KHz. The one > bit sample is decimated to 24 bits. This ADC is a fairly typical high end > stereo audio converter. > > The two approaches are very different and each fulfills a different need. > Fortuneately, the K4 can provide both with the right options installed. > > Sent from my iPad > >> On May 29, 2019, at 1:08 AM, Bill Frantz <[email protected]> wrote: >> >> A friend and I were discussing the K4HD and he said that it sounds a lot >> like the K3S. Well, we both agreed that since the K3S has world class >> receiver specs, that would not be a bad thing. >> >> I started comparing the information about the K4HD with the K3S manual and >> found: >> >> K3S K4HD Feature >> 8.215MHz ~8MHz First IF >> 15KHz ? Second IF (if used in the K4) >> 24 16 ADC width >> ~30KHz? 122MHz ADC sample rate (WAG for the K3S) >> 5 2+1 Crystal filters (one empty slot is from a post on this >> list.) >> >> Note: In the K4 FAQ there are forward references to a K4HD section, but I >> couldn't find it on the web site. >> >> Analysis and wild speculation >> >> Both DSPs and ADC technology have had at least 10 years to improve between >> the K3S and the K4. WHile I don't track ADC tech, DSPs are computers and we >> all know what has happened in computers. The basic processors are somewhat >> faster, and there are many more cores on a chip. The K4 certain to have >> taken advantage of these improvements. >> >> The K3S uses a 32 bit floating point DSP, and I don't see any reason to >> change that specification for any of the K4 models. Not changing means that >> much of the K3 DSP code should easily port to the new DSP. >> >> I don't understand how the basic K4 can get good dynamic range with a 16 bit >> DAC. The K3's 24 bit DAC seems a better choice, although getting high speed >> and wide bit width at the same time is hard and expensive. >> >> In the wild speculation department, there seem to be several approaches for >> the K4HD. (1) Run a K3 like superhet with an ~8MHz IF and digitize that into >> the DSP. (2) Duplicate the K3's 2nd IF and use the 16 bit ADC at 15Khz. (3) >> Use a wider ADC at either 8MHz or at 15KHz. Note that one of the features of >> the K4 is ease of upgrade to new ADCs. >> >> 73 Bill AE6JV >> >> --------------------------------------------------------------------------- >> Bill Frantz |"Web security is like medicine - trying to do good for >> 408-356-8506 |an evolved body of kludges" - Mark Miller >> www.pwpconsult.com [1] | >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Links: ------ [1] http://www.pwpconsult.com ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
